Duct gas analyzers which are mounted in ducts or flues typically include a source unit containing a source of suitable radiation, such as infrared, ultraviolet and visible light, mounted on one side of the duct or flue. A detector unit containing sensing devices for separating and measuring the specific wave lengths for analysis of the duct gases is mounted on the opposite side of the duct or flue. The source unit and the detector unit are typically connected by some member, such as a pipe, which serves both as an alignment fixture and as a sample cell, with apertures in the pipe allowing the duct gases to flow through the sample cell, which extends across the path of flow of the gases.
Windows of various types are typically used to isolate the source and detector units from the duct gases to protect the source, detectors and their associated electronic components. Additionally, fresh air is normally brought into the sample chamber and passed over the window faces to provide some cleansing action and to maintain a clean air region between the duct gases and the window. While this fresh air helps to keep the windows clean, over an extended time period deposits from the duct gases, particularly when these gases are gases of combustion from a furnace, build up on the window and cause errors in the qualitative and quantitative determination of the duct gas constituents. One method of correcting for this problem, caused by the deposit build-up is to block the apertures through which the duct gases enter and leave the sample cell to allow the fresh air introduced from outside to clean or purge the cell of the duct gases. During this purging the signal from the detectors may be measured to provide an indication of the error signal being caused by the deposit coating on the windows.
In one type of prior art apparatus the sample cell is in the form of a pipe having apertures aligned with the gas flow and having a pair of doors hingedly mounted to the sample cell to close off those apertures when swung to their closed position against the cell and to open those apertures when swung back away from the cell. This prior art apparatus, while providing advantages over those sample cells having no means for closing the apertures, possesses a number of disadvantages itself. For example, the doors require a relatively tight seal to achieve effective purging, a seal which is difficult to achieve and maintain over time, particularly when exposed to high temperature combustion gases. Additionally, in applications where there are large quantities of particulate matter in the duct gases, a build-up of the deposits around the door openings can prevent the doors from closing to exclude the duct gases from the chamber for purging. Such an apparatus also requires, in large scale applications, high powered fans to provide sufficient fresh air to clear the chambers. Additionally, where such sample chambers are mounted through an aperture in the duct wall, which aperture is only slightly larger than the cross-section of the closed sample chamber, a malfunction of the door closing apparatus may create sufficient mechanical interference to preclude removal of the sample chamber for service without shutting down the furnace, boiler, or other apparatus connected with the duct.
In fluid mechanics there are well known techniques for determining the pressure distribution of a flowing stream of gases at various points over a body immersed in that stream. However, none of the prior art known to the applicant has taken advantage of the kinetic energy in such a stream to assist in the operation, and particularly the purging, of a gas analyzer sample chamber.